Fix comment references to "_unittests.cc" files (base/)

Following up on:
https://chromium-review.googlesource.com/c/chromium/src/+/7510296

Update comments in various files incorrectly referred to unit test files
with a plural "unittests.cc" suffix.

#cleanup

Bug: None
Change-Id: Ieb9af728b92ea70e2ea46aacd8d5d8c9a209a8f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7522888
Commit-Queue: Andrew Williams <awillia@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1577260}
NOKEYCHECK=True
GitOrigin-RevId: cdfed01946fc25d97b4379bda0ccbb5d680d0771